Method and system for preparing samples for gas chromatography

ABSTRACT

The invention relates to a method and device for preparing samples for gas chromatography, the sample material being guided through a liquid chromatograph ( 1 ) by means of a solvent and being monitoring by means of a detector ( 2 ), and, on the basis of the detector ( 2 ) recognizing a chromatographic region of interest, the sample material of this region being removed from the stream of sample material leaving the liquid chromatograph ( 1 ) and being analyzed by gas chromatography. In this case, the stream of sample material leaving the liquid chromatograph ( 1 ) is guided through a flow-through cuvette ( 4 ), and the removal is carried out, until the end of the chromatographic region of interest, via a syringe ( 18 ) which, when the sample material of the region of interest reaches the flow-through cuvette ( 4 ), is introduced into the stream of sample material guided through the flow-through cuvette ( 4 ) and is drawn upwards at a predetermined ratio to the transport rate of the stream of sample material.

BACKGROUND OF THE INVENTION

The invention relates to a method and a system preparing samples for gaschromatography.

In order to prepare samples for gas chromatography, it is known toconnect a liquid chromatograph (LC) upstream of the gas chromatograph(GC). For this purpose, in on-line operation, a valve circuit with fixedbypass injector is used. Immediately after passing an LC detector, thesample material is removed from the LC and conveyed via a transfer lineto a complicated valve device which is connected in such a way that thesample material contained in the bypass injector is flushed directlyinto a gas-chromatography capillary column using a carrier gas. However,such columns have only a limited sample capacity, so that this operationis possible only by employing complicated coupled-column techniquecomprising up to three series-connected capillary columns, firstly agas-chromatographically inactive column for holding the sample material,then a precolumn for blending out the solvent, and finally a main columnfor the chromatographic separation. A so-called early-vapor exit issituated between the latter two columns. In this case, the bypassinjector can only be changed by means of costly modification of thedevice. There are not possibilities for variation, and only the start offeed to the inactive column, and hence the selection of the startingpoint of a chromatographic region of interest, can be influenced.

In off-line operation, the sample material, downstream of the LC, is insample bottles of a fraction collector, in order then to be analyzedfurther in a conventional manner.

SUMMARY OF THE INVENTION

The object of the invention is to provide a method and a system forpreparing samples for gas chromatography, the sample material beingguided through a liquid chromatograph by means of a solvent and beingmonitored by means of a detector, and, on the basis of the detectorrecognizing a chromatographic region of interest, the sample material ofthis region being removed from the stream leaving the liquidchromatograph and being analyzed by gas chromatography, characterized inthat the stream of sample material leaving the liquid chromatograph isguided through a flow-through cuvette, and the removal is carried out,until the end of the chromatographic region of interest, via a syringe,which is introduced into a flow-through cuvette and is drawn upwards ata predetermined ratio to the transport rate of the stream when thatregion of the sample material which is of interest has reached theflow-through cuvette.

A feature of the invention is that it allows simple and variablepreparation of samples.

As a result, the quantity of sample material which can be used for gaschromatographic analysis is variable and not constant. The removal ofthe sample material which is to be analyzed by gas chromatography isboth very simple and the apparatus is simple.

Further configurations of the invention can be found in the followingdescription and the subclaims.

The invention is explained in more detail below with reference toexemplary embodiments which are illustrated in the appended drawings, inwhich:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of a device for preparing samples for gaschromatography,

FIGS. 2a and 2 b are cross-sectional view of a further embodiment.

DETAILED SPECIFICATION

The device illustrated comprises a liquid chromatograph 1 with adetector 2, liquid sample material, substances dissolved in a solvent,being conveyed through the liquid chromatograph by means of a pump,passing by the detector 2 and after passing the detector 2 being outputinto a preferably flexible, transfer line 3, which in order to avoidfalsification of the results may be deactivatable, so as to be inactivewith respect to substances in the sample material flowing through. Thetransfer line 3 is connected to a collection vessel (not shown) via aflow-through cuvette 4.

In the exemplary embodiment illustrated in FIG. 1, the flow-throughcuvette 4 is formed by a cuvette body 6 which is provided with athrough-bore 5, a transverse bore 7 opening out into the through bore 5.The transfer line 3 opens out into the lower end of the through-bore 5,while the transverse bore 7 is connected to a discharge line 8 whichleads to the collection vessel. The line 3, 8 and the bore 5, 7expediently have narrow, constant internal diameters, in such a mannerthat there is virtually no cross-mixing of the sample material conveyedtherein.

At the top, the cuvette body 6 in accordance with FIG. 1 has a septum 10which is clamped in by means of a screwed-on mouthpiece 9. As analternative, as illustrated in FIGS. 2a, 2 b, the cuvette body 6, or anattachment 6 a thereon, may also include a valve body 11 which closesoff the bore 5 at the top, and in a guide 12, in a spring-loaded mannerwith respect to a stopper 13, has an end face which runs at an angle tothe bore 5 and with respect to which the sealing surface is set back. Inthis case, in the closed position the sealing surface interacts with anannular stop surface 14, which surrounds the bore 5, in the attachment 6a.

Before commencing operation, the flow-through cuvette 4 is flushed withinert solvent in the unsealed state, in order to eliminate any airpockets. In operation, sample material then flows out of the liquidchromatograph 1, through the transfer line 3, the flow-through cuvette 4and the discharge line 8, into the collection vessel, while the inertsolvent stands in the region of the bore 5 between the transverse bore 7and the septum 10 or the valve body 11.

The device furthermore comprises an insertion device 15, whichcomprises, for example, a frame 16 with a vertically displaceableholding carriage 17 for a syringe 18. The holding carriage 17 has amount 19 for vertically holding the syringe 18 with the needle 20pointing downwards and can be moved up and down by a predetermineddistance by means of a drive (not shown) in such a manner that theneedle 20 of the syringe 18 can be introduced into the bore 5 eitherthrough the septum 10, or, by pressing back the valve body 11, past thevalve body, and approximately beyond the opening of the transverse bore7.

The syringe 18 contains a piston 21, which is connected to a piston rod23 which can be guided outwards and on the end side has a head 22. Adraw-up device 24, which may comprise, for example, a gripper 24 a, actson the piston rod 23, which gripper can be used to pull the piston rod23 upwards with respect to the stationary syringe 18, so as to draw thesyringe 18 upwards, by means of a drive which is part of the insertiondevice.

The detector 2 is coupled to a computerized control system 25, which canbe used to input the start and end of a chromatographic region of thesample material, the beginning of which is to be established by thedetector 2 and the length of which depends on the region of interest ofa particular chromatogram.

When the detector 2 detects the beginning of the chromatographic regionof interest of the sample material, it uses the control system 25 tomove the syringe 18 into the flow-through cuvette 4. Since the rate atwhich the sample material is conveyed through the liquid chromatograph 1and the flow-through cuvette 4 is known, owing to the pump capacity andthe cross-section of flow, it is also known when the region of interestwill appear in the flow-through cuvette 4, so that the syringe 18 isintroduced into the flow-through cuvette 4 in accordance with this time.

Then, the syringe 18 is drawn upwards, via the control system 25, at aconstant rate which is predetermined by the control system 25. Normally,this takes place in accordance with the stream of sample material, sothat all the sample material within the chromatographic range ofinterest is taken up by the syringe 18. However, if the region selectedis too large, so that the syringe 18 cannot hold all the sample materialconveyed in the associated time period, this can advantageously becalculated by the control system 25 and indicated to the user, so thateither the region is limited in a suitable manner by freshly inputtingits beginning and end, if possible, or else sample material istransferred in a split ratio into the syringe 18, while the remainingpart flows past the needle 20 of the syringe 18 and into the dischargeline 8. In this case, the draw-up rate is reduced by the factor of thesplitting ratio. This ensures that sample material is drawn up throughthe syringe 18 in accordance with the representative chromatogramsection of interest. In this case, the needle 20 is sealed in a gastightmanner with respect to the outside by means of the septum 10 or anotherseal 26 between mouthpiece 9 and cuvette body 6.

Expediently, the flow-through cuvette 4 may be mounted on a turntable 27which has a cutout via which the syringe 18, after the samplepreparation has taken place, can feed its contents into a feed device 28of a gas chromatograph 29 which has a capillary column. The feed devicemay, for example, be a cold feed device 28, for example in accordancewith EP-0,451,566-A2 or in accordance with DE-196 53 406 C1. It isexpedient for solvent to be blended out first, after which the samplematerial is transferred, by means of carrier gas, to the capillarycolumn of the gas chromatograph 29 and is analyzed.

If appropriate, a plurality of flow-through cuvettes 4 may be arrangedon the turntable 27.

It the solvent used for the liquid chromatography, for example water, isunsuitable for the gas chromatography, the contents of the syringe 18may initially be placed in a sample bottle, where they are redissolvedin a solvent which is suitable for the gas chromatography, for examplehexane, so that two separate layers, situated one above the other, areformed, of which the layer which is for gas chromatography is removedagain and is fed into the gas chromatograph 29 by means of a syringewhich is introduced into the feed device 28.

The present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof, and it istherefore desired that the present embodiment be considered in allrespects as illustrative and not restrictive, reference being made tothe appended claims rather than to the foregoing description to indicatethe invention scope of the

In the claims:
 1. A method for preparing samples for gas chromatographycomprising the steps of: a) guiding sample material through a liquidchromatograph with a detector by a solvent; b) discharging the samplematerial from the liquid chromatograph in a stream at a transport ratethrough a flow-through cuvette; c) recognizing by way of the detector achromatographic region of interest in the stream; and d) withdrawing thechromatographic region of interest in the stream of the sample materialinto a syringe at a predetermined ratio to the transport rate of thestream when that region has reached the flow-through cuvette.
 2. Themethod according to claim 1 further comprising the step of placing thesample material removed by the syringe in a sample bottle where it isredissolved in a solvent which is suitable for the gas chromatography.3. The method of claim 1 further comprising the step of feeding thesample material removed from the syringe by a carrier gas via a coldfeed device to a gas chromatography column.
 4. The method according toclaim 3 further comprising the step of placing the sample materialremoved by the syringe in a sample bottle where it is redissolved in asolvent which is suitable for the gas chromatography.
 5. The method ofclaim 3 further comprising the step of discharging the solvent in thesyringe before the sample material is fed into the gas chromatographycolumn.
 6. The method according to claim 5 further comprising the stepof placing the sample material removed by the syringe in a sample bottlewhere it is redissolved in a solvent which is suitable for the gaschromatography.